1. Field of the Invention
The present invention relates to a lead frame for a semiconductor package having high molding compound adhesiveness.
2. Description of the Related Art
A lead frame together with a semiconductor chip constitutes a semiconductor package. The lead frame facilitates connection of the semiconductor package to an external terminal (e.g., a PCB) and also supports the semiconductor chip.
FIG. 1 is a plan view illustrating a typical lead frame for a semiconductor package. Referring to FIG. 1, a lead frame 1 includes a die pad 2 and a plurality of leads connected thereto. The die pad 2 is connected to a pad supporting unit 3 by a rail 7 and supports a semiconductor chip (not shown).
The plurality of leads includes a plurality of inner leads 4 and a plurality of outer leads 5. A dam bar 6 that maintains a gap and supports the gap is formed between the inner leads 4 and the outer leads 5. The dam bar 6 and the rail 7 are removed after the assembling of the semiconductor package is completed.
A lead frame such as one having the above-described structure together with a semiconductor chip (e.g., a memory device) constitutes a semiconductor package through an assembling process. An example assembling process includes a die attaching process, a wire bonding process, and a molding process. The die attaching process is a process for attaching a semiconductor chip (die) to a pad of a lead frame, the wire bonding process is a process for connecting by bonding a terminal unit of a semiconductor chip to an inner lead of a lead frame using a metal, such as gold, and the molding process is a process for sealing a chip, wires, and inner leads using an insulating material, such as a molding resin, for example, epoxy molding compound (EMC).
To increase the adhesiveness of the semiconductor chip to the pad in the die attaching process and to improve the capability of bonding wires to the inner leads 4 in the wire bonding process, a metal having predetermined characteristics can be coated on the die pad 2 and the inner lead 4. Also, to improve the solder wettability of the outer leads 5 in the assembling process after the molding process, a soldering base can be plated on a predetermined portion of the outer leads 5 using, for example, a Tin-Lead (Sn—Pb) alloy.
However, the soldering base plating process is complicated and exposed Lead (Pb) and a Pb plating solution can cause an environmental problem. Also, a process for removing non-uniformity of the plating layer is required in the process of the soldering base plating. Also, the failure of a semiconductor chip occurs due to the penetration of the plating solution between a surface of the lead frame and the molding resin.
To solve the above problems, a pre-plated frame method has been proposed. In this method, the Pb plating process can be omitted in a subsequent process by pre-plating a material having high solder wettability on a metal upper layer before performing the semiconductor packaging process. A lead frame that has pre-plated using the pre-plated frame method draws attention due to a simple subsequent process and the reduced environmental problem by omitting Pb plating in the semiconductor packaging process.
FIG. 2 is a cross-sectional view illustrating an example of a lead frame manufactured using a conventional pre-plated frame method. Referring to FIG. 2, a Nickel (Ni) plating layer 12 is entirely formed on the base metal layer 11, which contains Cu as a main component, and a Palladium (Pd) plating layer 13 is directly formed on the Ni plating layer 12. That is, Ni and Pd are sequentially plated on the base metal layer 11.
When a lead frame having an uppermost Pd layer is used, an environmental problem of exposed Pb is avoided and a semiconductor packaging process can be simplified. However, due to heat generated from the semiconductor assembling process the Pd plating layer 13 forms an oxidized Pd compound, which may adversely affect physical properties (e.g., electrical conductivity, adhesiveness, etc.) of the Pd layer 13. In particular, oxidation of the Pd plating layer 13 reduces interface adhesiveness (wire bonding capability) and solder wettability between a gold wire and the lead frame. Also, when the Pd plating layer 13 absorbs hydrogen during plating, the Pd plating layer 13 becomes weakened/embrittled and made susceptible to cracking due to an impact.
Various lead frames to solve the foregoing problems have been proposed in U.S. Pat. No. 6,469,386. Two such lead frames are illustrated herein as FIGS. 3A and 3B. In one lead frame illustrated in FIG. 3A, a Ni plating layer 22, a Pd layer 23 and a Gold (Au) plating layer 24 are sequentially formed on a base metal layer 21. In another lead frame illustrated in FIG. 3B, a Ni or Ni alloy plating layer 22′, a Pd or Pd alloy plating layer 23′ and an Au—Pd alloy plating layer 24′ are sequentially formed on the base metal layer 21. The structure of the plating layers 22, 23, 24 and 22′, 23′, 24′ is substantially the same as the structure depicted in FIG. 2 with the addition of the uppermost Au plating layer 24 or the Au—Pd alloy plating layer 24′.
The oxidation resistance of Au is greater than Pd. Therefore, as depicted in FIG. 3A, when a pure Au plating layer 24 is formed on the uppermost part pf a lead frame, the Au plating layer 24 prevents the Pd plating player 23 from being oxidized during a semiconductor packaging process in which a thermal process is performed, thereby solving the conventional low wire bonding capability and solder wettability problems.
However, disadvantageously, a molding resin generally has low affinity to a surface of a pure metal or alloy. Further, it is known that, in comparison to a surface of pure metal or alloy, when an oxide layer is formed on the surface of the pure metal or alloy, the adhesiveness of the molding resin is improved. Therefore, when a pure Au plating layer is formed on a contact surface of the molding resin as an oxidation preventing layer for Pd, the adhesiveness of the molding resin is reduced.
One known way to improve adhesiveness of the molding resin to the metal surface is depicted in FIG. 3B, when the Au—Pd alloy plating layer 24′ composed of Au and Pd is formed on the Pd or Pd alloy plating layer 23′, the adhesiveness between the molding resin and the Au—Pd alloy plating layer 24′ is increased by the oxidation of exposed surface Pd portions.
Recently, the method of manufacturing an environment-friendly semiconductor package draws attention. To manufacture an environmental problem-free semiconductor package, high adhesiveness between a lead frame and the molding resin is required under a severe atmosphere in which temperature and humidity are very high. However, under the severe atmosphere, the Au—Pd alloy plating layer 24′ has poor adhesiveness with the molding resin.
That is, as it will be described later, according to the result of a moisture sensitivity level (MSL) evaluation through a coupon test after passing 168 hours under an atmosphere of temperature of 85° C. and a relative humidity of 85%, the severe delamination of layers is observed and the adhesiveness of the molding resin is reduced. Therefore, the adhesiveness of the molding resin is degraded under a severe atmosphere, such as a high humidity condition.